Projection technology is becoming wide spread thanks to rapid technology evolution and more efficient production techniques. Reference is made to the book “Projection Displays”, E. H. Strupp and M. H. Brennesholtz, Wiley, 1999.
Current commercially available projectors can only project one computer or data image in its native resolution if it is a high resolution image such as for example SVGA (Super VGA or Super Video Graphics Array or Adaptor, 800*600 pixels), XGA (Extended Graphics Array or Adaptor, 1024*768 pixels), 16:9 HDTV standard format (1280*720 pixels), SXGA (Super XGA, 1280*1024 pixels), Wide XGA (1365*768 pixels), SXGA+ (1400*1050 pixels), UXGA (Ultra XGA, 1600*1200 pixels), 16:9 HDTV standard format (1920*1080 pixels), 16:10 widescreen PC displays (1920*1200 pixels), 16:9 European HDTV format (2048*1152 pixels) or Quad XGA (2048*1536 pixels).
Reference is made to the book “Display Interfaces”, R. L. Myers, Wiley, 2002, for explanation of these terms.
When it is desired to project multiple computer or data images on one screen in native resolution, e.g. images from two laptop computers in order to be able to compare them, then often one projector per displayed image is used. This solution is expensive because of the cost of supplementary projectors. Furthermore, it brings along additional problems, the most important of which are geometrical alignment of the different images and colour adjustment of these images projected by different projectors.
The geometrical alignment forms a problem because each projector has its own projection lens with its own geometrical aberrations or distortions. Projecting a plurality of such distorted images adjacent each other always involves some bad alignment, as represented for example in FIG. 1, which shows two images 2, 4 projected adjacent each other onto a projection surface 6 by two different projectors. Each of the images 2, 4 is distorted.
The book by Myers above proposes a hypothetical solution to the problem of displaying images of different resolution on a single display by means of conditional updating. That is each sub-image has to be updated separately rather than the complete screen as an entity.
When images of different projectors are combined, furthermore these images need to be colour adjusted so that originally colour matched images look the same, since the projected image of each projector has a slightly different colour point and colour uniformity. Even with the best possible known colour compensation schemes, one can always notice a visible difference.
Because of colour drift of projectors over time, this difference can get bigger over time.
A solution for these problems has been proposed by a device according to U.S. Pat. No. 5,153,621. With this device, a pair of discrete images are disposed in adjoining relation on a view screen to form a single image thereon with minimal or no illuminance discontinuity at their juncture. To accomplish this, there is provided a light source, a condensing lens, a mirror system, a pair of discrete image displays, and a projection lens. The lens and mirror system project light rays from the source along discrete optical paths for passage separately through each image display. A single projection lens defining an optical axis is provided for projecting the images in adjoining relation on a view screen. Interposed between the image displays and the projection lens is an optical system including a lens and mirror for combining the light rays passing through the image displays along discrete optical paths for passage through the projection lens such that a single image combining the images from the discrete image displays in substantially seamless adjoining relation is formed on the view screen.
The above device has the disadvantage that the two image displays need to be aligned very accurately. It is known that such mechanical adjustments of two image displays, LCD panels for example, diverge under temperature cycling circumstances. Furthermore, if the characteristics of the image displays for forming the adjacent projected images are different, then a join seam becomes visible on the screen. This join seam is caused by a difference between the characteristics of the image displays, and accordingly the quality of the projected image deteriorates. Making two exactly identical image displays is difficult (in reality impossible) due to process differences. This is especially true for grey levels on the image displays, such as LCD's for example. Furthermore, the seam is also visible because the light beam is split into two parts, one for each panel, resulting in illuminance and colour non uniformity.